In the field of large scale power generation from fossil-fired steam electric power plants, considerable uncertainty exists as to the properties and quality of the fuels that will be fired during the life of the plant. When designing pulverized coal-fired power plants, the boiler size and configuration is often selected on the basis of the lowest grade coal that may be fired by the plant in the future, with the result that an oversized and costlier boiler is specified than will probably be needed.
Also, to reduce emissions from the pulverized coal-fired boiler under the worst possible fuel assumption, the equipment selected for the reduction of particulates, sulfur and nitrous oxides emissions is correspondingly oversized and costlier than will probably be needed.
A variety of Fluidized Bed boilers are currently being developed for firing coal, particularly the lower grades. Such Fluidized Bed boilers are inherently low emitters of particulates, sulfur and nitrous oxides; therefore they require simpler and less costly emissions control systems. However, these Fluidized Bed boiler designs have not been demonstrated for the generation of the high pressure and temperature superheat and reheat steam that is required for efficient electric power generation. Also, questions persist regarding the practicality, economic viability and competitiveness of Fluidized Bed boilers for large power plants vs. Pulverized Coal-fired boilers, even with the added expensive emissions control systems for the latter.
Another alternative (from the Pulverized Coal-fired and the Fluidized Bed boiler choice) is firing higher quality or cleaned coal in a Pulverized Coal-fired boiler. This in most cases will result in higher cost fuel, as well as presenting problems with coal waste, and waste stream disposal from the cleaning plant.
Added impetus to the above fuel and firing equipment selection problems in power generation is provided by situations where existing power plants with boilers designed for a certain type of fuel have to be modified, and the fuel is often altered. Such situations exist when power plants with boilers designed to be fired with oil or natural gas are converted to utilize coal instead, or older coal-fired plants are rehabilitated and their life extended, but changing fuel properties or emissions requirements render the existing Pulverized Coal-fired boilers deficient for the purpose of providing the required steam flow to meet maximum plant output as defined by other plant equipment. A large scale such situation will occur when existing coal-fired power plants in the Mid-West currently firing relatively high sulfur coal will be required to drastically curtail emissions, primarily for the purpose of reducing acid rain deposition in the Northeast United States and the Canadian Maritime Provinces.